The fridge I have is a full size fridge. I thought about using the AC but I don't want to deal with the condensate from it running or the noise. I have pretty much talked myself into using the fridge (keeping the model and serial number tag so that I can order a part if anything goes bad and needs replaced) unless a more compelling argument is made.
What I've found is that running a mini-fridge in an extended fermentation chamber at between 55 and 70F leads to condensation issues inside the chamber, and hence mold. I've had to add a drain to the tray that sits under the cooling part (evaporator), routed to a gallon jug, but I haven't managed to get the system level enough to drain the tray properly yet.
With an AC unit, the condensate should be routed to the outside portion of the unit, and then to a drain plug, which you could connect to a tube and bucket outside of the chamber. Some of the condensate will be used by a slinger fan to help cool the hot side of the AC unit. The AC unit will dry the inside of the chamber quite effectively, so mold shouldn't be an issue.
I'm also wishing I had another fridge to use for lagering and/or serving cask beer at lower temperatures than the fermentation chamber that I can use at the same time. I'm quite tempted to rebuild the chamber with an AC unit, and put the fridge back together as a cask beer cellar/lagering fridge (Haven't made anything more lagerish yet than a steam beer because I can't reach true lager temps).
It seems to me that AC units are optimized for cooling a large volumes to 65-75F with significant air exchange and drying the air, while fridges are optimized for cooling a small fairly well sealed volume to 35-45F. Keeping their new uses somewhere close to that is probably a good plan. A larger fermentation chamber is somewhere between the two original applications. However, AC units are probably all rather massively over-specced for a reasonable* fermentation chamber, and so probably less efficient. Not sure what the right way to go is.
*If you have a 1 bbl system and 12 tap keezer, your definition of reasonable may be different to mine.